The present invention relates to a master cylinder for supplying a brake fluid to a brake system of a vehicle and a method of manufacturing the master cylinder.
As a master cylinder for supplying a brake fluid to a brake system of a vehicle, for example, as described in Japanese Patent Publication No. 2006-123879, there is provided a constitution in which a piston is operatively provided on an inner circumference of a cup seal held in a seal groove of a cylinder main body so as to be slidable therein, and a brake fluid in a pressure chamber formed by the piston and the cylinder main body is pressurized by the sliding movement of the piston so as to be supplied to the brake system. This type of master cylinder has an annular wall 102 in a cylinder main body 100, which is a part of a seal groove and is located behind a cup seal 101, and a corner portion provided between the annular wall 102 and a cylinder wall 104 facing an outer circumference surface of a piston 103 is formed of a chamfered portion 105 having, for example, a circular arc shape in a cross-section along a diameter direction (i.e., a cross-section taken along a diameter direction passing through a center line) of the cylinder main body 100, as shown in FIG. 10. The chamfered portion 105 is formed such that tangents to both ends of the circular arc are in line with a wall surface of the annular wall 102 and a wall surface of the cylinder wall 104, respectively.
In the above-described master cylinder, when the piston 103 slides back in an arrowhead direction X shown in FIG. 10 while a high-level of hydraulic pressure being held in a pressure chamber, the cup seal 101 is likely to deform and intrude into a clearance between the piston 103 and the cylinder wall 104, wherein a stress concentration in a deformed portion 106 may be relieved by forming the chamfered portion 105. That is to say, if the chamfered portion 105 is not formed, the stress concentration in the deformed portion 106 could be so high as to damage the cup seal. In order to ensure the relief of the stress concentration, it was intended to increase a radius of curvature of the chamfered portion 105 into R2, which has been conventionally R1 as shown in FIG. 11. By this change, the maximum stress value S in the deformed portion 106 of the cup seal 101 was reduced as shown in FIG. 12A, which shows an increase in the radius from R1 to R2 has high effect on the relief of the stress concentration. However, the change involved a problem that, by increasing the radius of the chamfered portion 105 from R1 to R2, locations defining both ends of the circular arc of the chamfered portion 105 are shifted away from A1, B1 to A2, B2, respectively, resulting in a considerable increase in an intrusion L of the deformed portion 106, as shown in FIG. 12B, which is likely to causes more bite in the cup seal.